[time-nuts] Re: Phase coherence with 2x GPSDO

[Mattia Rizzi]

Hi,
You can definitely get sub-ns precision using a 4-8 MHz bandwidth wireless
protocol. You are not limited by the sampling period. In my previous work I
used 802.15.4-CSS (chirp based modulation), implemented by me in a SDR. The
sampling rate was 32 ns but with crosscorrelation and sample interpolation
you can get down to half ns precision (1 sigma)

See:
https://www.researchgate.net/publication/261329724_Timestamping_and_ranging_performance_for_IEEE_802154_CSS_systems





Il giorno lun 7 mar 2022 alle 19:39 Krishna Makhija <km5es at virginia.edu> ha
scritto:

> Thanks Mattia. What did you use for your Layer 1? I need to place one of
> the SDRs on a drone and one on the ground so a fiber or LAN cable is out. I
> could use WLAN but can you get sub-nanosecond performance over wi-fi? My
> initial guess would be no but I am not certain.
>
> Michael: I've had the same question but I can't see how it could
> possibly "know" its own frequency error/uncertainty. What would it
> reference to? I'll try asking the manufacturer anyway.
>
> Regards,
> Krishna
>
> On Mon, Mar 7, 2022 at 11:46 AM Mattia Rizzi <mattia.rizzi at gmail.com>
> wrote:
>
> > Hello Krishna,
> > what is your end application? How far away are those boards?
> > If each SDR can communicate to each other, you can run PTP over an ad-hoc
> > Layer 1.
> > I was able to get timestamps out of a 2.4 GHz chirp-based protocol with
> > less than 0.5ns RMS noise and two-way ranging error down to 10-20
> > centimeters, using two SDR.
> >
> >
> > cheers
> >
> > Il giorno dom 6 mar 2022 alle ore 23:48 Krishna Makhija <
> > km5es at virginia.edu>
> > ha scritto:
> >
> > > Hello Tom,
> > >
> > > Yes, the GPSDOs are working well. However, when I use each as a
> reference
> > > to a separate radio, I find there is a slow phase change over time
> > between
> > > said radios. I imagine this is expected since there will always be some
> > > error between two discrete oscillators. However, I am hoping to use the
> > PPS
> > > and FEE metadata to compute what the phase *should* have been in
> > > post-processing. So far, it is not working out for me. I am wondering
> if
> > > that is even possible or if my math is just wrong.
> > >
> > > Bob,
> > >
> > > The SDRs have an LO running at 150 MHz (~6.66 ns) so a PPS wander of +-
> > 10
> > > ns is >360 deg. With a common-mode reference I see a small phase change
> > (+-
> > > 3-4 deg) but that is not an option for my application.
> > >
> > > Where does the PPS offset come from? Isn't it from the positioning
> error?
> > > Typical GPS receivers have 1-3 m of positioning error which should give
> > > you +- 10 ns. Why is this a "dream" performance? It should be expected
> > from
> > > any modern GPS receiver.
> > >
> > > Thanks for your inputs so far.
> > >
> > > Krishna
> > >
> > > On Sun, Mar 6, 2022 at 4:30 PM Bob kb8tq <kb8tq at n1k.org> wrote:
> > >
> > > > Hi
> > > >
> > > > They *are* phase coherant to within 360 * 100/ 10 = 36 degrees. You
> > > > can get them to *maybe* ten degrees with this and that done here or
> > > there.
> > > >
> > > > If you want them within a degree, no you can’t do that directly with
> > GPS.
> > > > If your definition of phase coherent is zero degrees, a pair of
> SDR’s
> > > off
> > > > the
> > > > same buffered clock will have issues with that definition in the real
> > > > world
> > > > of temperature wandering around …..
> > > >
> > > > Bob
> > > >
> > > > > On Mar 6, 2022, at 2:04 PM, Krishna Makhija <km5es at virginia.edu>
> > > wrote:
> > > > >
> > > > > Hi Bob,
> > > > >
> > > > > I am currently getting +- 10 ns nominal. The antennas are currently
> > > > almost
> > > > > next to each other (roughly 1-2 inches apart). Yes, they should be
> > > > outside
> > > > > of each of their farfield zones. Here is the PPS offset I am seeing
> > > > during
> > > > > measurement:
> > > > > [image: image.png]
> > > > > And this is the frequency error I am seeing:
> > > > > [image: image.png]
> > > > > Overall, the GPSDOs seem to work pretty well. But the question
> still
> > > > > remains if one can hope to get them to be phase coherent, either in
> > > > > real-time or in post-processing.
> > > > >
> > > > > Jeremy: I bought these items by writing to them. I chose to place a
> > > > > purchase order (since I did it through my organization), but you
> > might
> > > be
> > > > > able to order by talking to them directly and paying using a credit
> > > card.
> > > > >
> > > > > On Sun, Mar 6, 2022 at 1:51 PM Bob kb8tq <kb8tq at n1k.org> wrote:
> > > > >
> > > > >> Hi
> > > > >>
> > > > >> How close are you trying to get?
> > > > >>
> > > > >> How far apart are the GPSDO’s?
> > > > >>
> > > > >> A “run of the mill” number would be out around 100 ns. A “pretty
> > good”
> > > > >> number is in the 20 ns range. A “crazy good” number would be 2 ns.
> > To
> > > > >> do better than this, you likely would need to go to a more exotic
> > > > >> configuration
> > > > >> on the GPSDO.
> > > > >>
> > > > >> Bob
> > > > >>
> > > > >>> On Mar 6, 2022, at 12:55 PM, Krishna Makhija <km5es at virginia.edu
> >
> > > > wrote:
> > > > >>>
> > > > >>> Hello,
> > > > >>>
> > > > >>> I am new to the whole precision time-keeping game (and to this
> > > mailing
> > > > >>> list) so I apologize in advance if my question is too naive or
> has
> > > been
> > > > >>> answered already in your mailing list.
> > > > >>>
> > > > >>> Is it possible to have two separate GPSDOs, each with their own
> > > > antennas,
> > > > >>> be phase coherent to each other? I have a Jackson-Labs Fury
> > > > >>> <https://www.jackson-labs.com/index.php/products/fury> and a
> > > Mini-JLT
> > > > >>> <https://www.jackson-labs.com/index.php/products/fury>. I am
> using
> > > > each
> > > > >> to
> > > > >>> provide a 10 MHz reference to two separate software-defined
> radios
> > > > >> (SDRs).
> > > > >>> In my tests I find that the phase offset between said SDRs has a
> > slow
> > > > >>> time-varying behavior. I know the frequency errors of the GPSDOs
> > are
> > > of
> > > > >> the
> > > > >>> order of parts per trillion which will show up as slow
> time-varying
> > > > phase
> > > > >>> offsets but I was hoping to use the PPS offsets and instantaneous
> > > > >> frequency
> > > > >>> errors that I get from these modules (using SCPI commands) to be
> > able
> > > > to
> > > > >>> "back out" or predict what that time-varying phase offset would
> be.
> > > Is
> > > > >> such
> > > > >>> a thing possible? Currently, the time-varying phase change does
> not
> > > > seem
> > > > >> to
> > > > >>> follow any discernible pattern and my attempts at backing out the
> > > phase
> > > > >>> change do not match my measurements.
> > > > >>>
> > > > >>> Here is the math I am using for calculating what I *think *the
> > phase
> > > > >>> *should* be:
> > > > >>> [image: image.png]
> > > > >>> [image: image.png]
> > > > >>>
> > > > >>> [image: image.png]
> > > > >>> Does any of this seem sensible? Any input is appreciated.
> > > > >>>
> > > > >>> TL;DR: Trying to get phase coherence between two separate GPSDOs
> > may
> > > > not
> > > > >> be
> > > > >>> possible but can you use PPS offsets and frequency errors
> metadata
> > to
> > > > >>> correct for it in post?
> > > > >>>
> > > > >>> Regards,
> > > > >>> Krishna
> > > > >>>
> > > > >>
> > > >
> > >
> >
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